1. Field of the Invention
The present invention generally relates to an optical element. More specifically, the present invention is directed to an optical element with employment of photonic crystals, refractive indexes of which are changed periodically, depending upon positions thereof.
2. Description of a Related Art
Very recently, photonic crystals have been effectively developed, which represent peculier dispersion characteristics, as compared with the conventional optical crystals. Thus, these photonic crystals are strongly expected to be applied to various sorts of optical elements such as optical modulation elements, optical deflection element, and switching elements. Since a photonic crystal is structured in such a manner that within a first material, second materials whose refractive index is different from that of the first material are arranged at an interval corresponding to a wavelength of light, this photonic crystal owns such a characteristic that the refractive index thereof is changed periodically, depending upon the positions of this photonic crystal.
The above-described characteristic represented by such a photonic crystal is described in detail, for instance, in the publication xe2x80x9cSuperprism phenomena in photonic crystalsxe2x80x9d written by H. Kosaka et al., in Physical Review B Vol. 58, No. 16, Oct. 15, 1998.
However, in the conventional optical elements with employment of the above-described photonic crystals, there are large optical reflection losses occurred in boundary surfaces between the photonic crystals and the normal media, which constitutes a problem to be solved, and also which has been revealed by the extensive analyses made by inventors of the present invention.
The present invention has been made to solve the above-explained problem and therefore, has an object to provide an optical element having a small light reflection loss produced in a boundary surface between a photonic crystal and the normal medium.
To achieve the above-described object, an optical element according to one aspect of the present invention includes: a normal medium region having a first refractive index; a photonic crystal region having a refractive index which is changed periodically depending upon a position thereof, the photonic crystal region having as an averaged refractive index a second refractive index different from the first refractive index; and an intermediate region interposed between the normal medium region and the photonic crystal region, the intermediate region having a refractive index which is gradually changed from the first refractive index to the second refractive index.
In accordance with the present invention, since the refractive index in the intermediate region is gradually changed from the first refractive index to the second refractive index, a mismatching problem of an impedance occurred between the normal medium region and the photonic crystal region can be gradually solved in the intermediate region. As a result, a reflection loss of light occurred in a boundary surface between the photonic crystal region and the normal medium region can be suppressed.